Password change as modal + admin can reset other user passwords

- New Modal.svelte component: overlay with backdrop click to close,
  title bar, reusable via children snippet
- Layout: password change moved from inline sidebar form to modal
  dialog. Clean UX with current + new password fields.
- Users page: 🔑 button per user opens modal for admin to set a
  new password (no current password required for admin reset)
- Backend: PUT /api/users/{id}/password (admin only, min 6 chars)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/Modal.svelte

@@ -0,0 +1,24 @@
+<script lang="ts">
+	import { t } from '$lib/i18n';
+	let { open = false, title = '', onclose, children } = $props<{
+		open: boolean;
+		title?: string;
+		onclose: () => void;
+		children: import('svelte').Snippet;
+	}>();
+</script>
+
+{#if open}
+	<!-- svelte-ignore a11y_click_events_have_key_events -->
+	<!-- svelte-ignore a11y_no_static_element_interactions -->
+	<div class="fixed inset-0 z-50 flex items-center justify-center bg-black/40" onclick={onclose}>
+		<div class="bg-[var(--color-card)] border border-[var(--color-border)] rounded-lg shadow-lg w-full max-w-md mx-4 p-5"
+			onclick={(e) => e.stopPropagation()}>
+			<div class="flex items-center justify-between mb-4">
+				<h3 class="text-lg font-semibold">{title}</h3>
+				<button onclick={onclose} class="text-[var(--color-muted-foreground)] hover:text-[var(--color-foreground)] text-lg leading-none">&times;</button>
+			</div>
+			{@render children()}
+		</div>
+	</div>
+{/if}